Crackly DSL40C

[x-pro]

I don't know how things are now with output transformer failures, but a long time ago there were cases of breakdown between turns of anode windings, due to burnout and destruction of the lacquer coating. In the past (a long time ago, in our country) winding was done in bulk, without sectioning. This does not necessarily cause the fuse to blow. But it is probably possible to check the anode current, of course without a signal.

[Timbo]

I would have started resoldering the sockets pins no matter how they looked. Can you put a meter or scope on a few places to see any spikes?

All socket pins have been resoldered. I can see the spikes with a scope on all nodes of the HT rail and at the power tube grids. Tbh, I’m not sure what that tells me?

[Timbo]

I don't know how things are now with output transformer failures, but a long time ago there were cases of breakdown between turns of anode windings, due to burnout and destruction of the lacquer coating. In the past (a long time ago, in our country) winding was done in bulk, without sectioning. This does not necessarily cause the fuse to blow. But it is probably possible to check the anode current, of course without a signal.

I’m not sure how I can test for this other than trying another transformer?

[The Dude]

When you remove the PI tube so that the amp is not crackling, do you still see spikes on the power rails? This might tell you if it's a PI circuit problem, or power supply to the PI circuit.

[Timbo]

When you remove the PI tube so that the amp is not crackling, do you still see spikes on the power rails? This might tell you if it's a PI circuit problem, or power supply to the PI circuit.

Good shout, I’ll check that out tomorrow.
thanks

[Timbo]

Valve bases are on order. No signs of tracking. I’m out now but will scope the PI grids tomorrow.

So I do see the noise at the PI input, with V1-3 removed, which is confusing me somewhat!

[Timbo]

Later ones have a half and full power in standby switch and older ones have triode pentode on the rear. Or that is my impression.

I guess the option is valve base. Any visual signs of HT tracking?

I guess other possibility is crackling is coming into the PI. Can you look or listen to the signal?

Sorry hard to read schematic just now.

I’m now suspecting either RL2b or RL3b contacts. I’ve now removed all of the preamp tubes and the PI. I can see the noise at LK3 (the preamp output), and I’ve disconnected R46. Does this sound reasonable?

[mikeydee77]

Might be worth lifting the coupling cap C47 to isolate that it is not coming from the valve base or pcb. HT can play funny tricks sometimes.

Do the relays respond to tapping in any way?

[Timbo]

Might be worth lifting the coupling cap C47 to isolate that it is not coming from the valve base or pcb. HT can play funny tricks sometimes.

Do the relays respond to tapping in any way?

I pulled C47 and the noise went away, both on the preamp output side and PI input side. Replaced C47 and the noise was back! Started probing for it again with the scope and I could see it on both sides of C47. Then it suddenly went away and I can’t bring it back. Probing, tapping etc. I think I’ll go ahead and replace those relays as that was where I’d traced it to before being led astray by C47! Do you happen to know a good source of those relays? Farnell seems favourite at the moment:

[mikeydee77]

Sorry not familiar with those parts. Farnell used to be good but rarely use these days due to their shipping charges.

[The Dude]

Are there not manufacturer part numbers on the relays themselves? If so, just Google search the number. Possibly these (GS-SH-205T)?

https://www.jameco.com/z/GS-SH-205T-...lgjfUNzb4mtw2B

[Timbo]

[QUOTE=The Dude;n1005035]When you remove the PI tube so that the amp is not crackling, do you still see spikes on the power rails? This might tell you if it's a PI circuit problem, or power supply to the PI circuit. [

so I think I may have been barking up the wrong tree with the relays.
I’ve now found that I can see and hear the noise when I probe the PI anode connections at the tube base with the PI tube pulled. What seems really odd is that I only hear it when I put the probe on the pins. It’s completely silent with the probe removed.

[mikeydee77]

Did you say that you replaced the plate resistors on the PI ? If not then, worth a shot...

Some thoughts...
Is C47 lifted? would help to isolate this stage. Also I wonder if you ground the PI grids would you get the noise?

I guess what is happening is that when the valve is present then current flows est. 1mA approx and when you have your scope probe a tiny current flows, that would uA surely, depends on your probe/scope. Got to suspect the Valve Base, Solder connection and also the other resistors on the cathode of the PI. Might be interesting to see what the noise level when probing with other things live a DVM, insulated tool. The other possibility is PCB material going bad like on blackstars.

[Timbo]

Did you say that you replaced the plate resistors on the PI ? If not then, worth a shot...

Some thoughts...
Is C47 lifted? would help to isolate this stage. Also I wonder if you ground the PI grids would you get the noise?

I guess what is happening is that when the valve is present then current flows est. 1mA approx and when you have your scope probe a tiny current flows, that would uA surely, depends on your probe/scope. Got to suspect the Valve Base, Solder connection and also the other resistors on the cathode of the PI. Might be interesting to see what the noise level when probing with other things live a DVM, insulated tool. The other possibility is PCB material going bad like on blackstars.

Ok, thanks. I may just have to start replacing components in order of probability, starting with valve bases.
Can a bad power transformer cause this kind of issue?

[The Dude]

I had originally discounted the theory because the noise disappeared when the PI tube was removed. With this new information, I now wonder if possibly one of the PI to output coupling caps is arcing.